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Chapter 1 – A Brief Summary of the Atmosphere Spring 2017 Meteorology 301 Atmospheric Science ● ● ● ● Meteorology: Atmospheric Phenomena and their time dependent behavior. Climatology: Long-term statistical properties Weather: Day to day variation in atmospheric conditions. Famous quote: “Climate is what you expect, weather is what you get”. Meteorology 301 Meteorology 301 Meteorology 301 Meteorology 301 Composite 500 mb Heights - DJF Meteorology 301 Composite 500 mb Heights - JJA Meteorology 301 Meteorology ● Physical: Atmospheric structure and composition. – ● ● Radiation, waves, clouds and precipitation formation. Synoptic: Description, analysis, and forecasting of large atmospheric motions. Dynamic: Description of atmospheric motions and their evolution based upon principles of fluid dynamics. Meteorology 301 Earth System ● ● Atmosphere: Gaseous Constituents. Hydrosphere: Total mass of water substance on or above the Earth's surface. – Cryosphere ● Biosphere: All animal and plant life. ● Lithosphere: The Earth's crust. Meteorology 301 What gases make up the atmosphere? Meteorology 301 Atmospheric Concentration Meteorology 301 Atmospheric Composition ● ● By mass – Nitrogen: ~ 76% – Oxygen: ~ 23 % – Argon: ~ 1% By volume – Nitrogen: ~78% – Oxygen: ~21% – Agron: ~1% Meteorology 301 Distribution of Mass ● Gravitational force. ● Pressure = force/area. ● – 1 N/m2 = 1 Pascal (Pa). – 100 N/m2 = 1 millibar (mb). Mean atmospheric pressure – MA go / 4 π RE2 ≈ 9.88x104 N/m2 = 988mb Meteorology 301 Vertical Profile of Pressure and Density ● ● ● Variability in vertical is much larger than the horizontal and temporal variability. In the lowest 100km, the logarithm of pressure drops off nearly linearly with height. Derivation Meteorology 301 Vertical Profile Meteorology 301 Composition as function of height ● Molecular diffusion and mixing ● Diffusion – Mean molecular weight of mixture gradually decreases with height. – Only lightest gases are present at higher levels. – Each gas behaves as if it were alone. – Density drops of exponentially with height ● Scale height ~ 1/M Meteorology 301 Composition cont. ● Rate of diffusion proportional to mean free path (mixing length). – ● Mean free path: average distance a particle must travel before colliding with another particle. Turbulent mixing – Does not depend on molecular weight. – Composition tends to be independent of height. Meteorology 301 Which one dominates? ● Lower atmosphere: turbulent mixing. – Small mean free path. ● Near 100km: diffusion and turbulent mixing. ● Upper atmosphere: diffusion – Large mean free path. Meteorology 301 Variable Constituents ● Water vapor and ozone are variable in space and time. ● Water vapor ● – Primary source: evaporation from surface. – Primary sink: condensation in clouds. – Typical lifetime: 1 week. – Concentration is largest near the ground. Ozone – Photochemical reactions between 20km-60km. – Destroyed at surface by reacting with plants and dissolving in water. – Very stable at 10km-25km with a lifetime on the order of a month. Meteorology 301 Temperature Distribution Meteorology 301 Troposphere ● Surface-200mb (0-10km). ● 80% of mass. ● Virtually all water vapor, clouds, and precipitation. ● Strong vertical mixing. ● “mean residence time”: few days to a week – Rapid vertical mixing and “scavenging” of aerosols by precipitation. Meteorology 301 Stratosphere ● 50mb-1mb (20km- ~50km) ● Small vertical mixing. ● Mean residence time : year or longer. ● ● Together with troposphere accounts for 99.9% of mass. Warming with height? Why? Meteorology 301 Mesosphere and Thermosphere ● ● Mesosphere – 0.5mb-0.01mb (~55km – 80km) – Temperature decreases with height. Thermosphere – Above 0.0001mb (Above 90km) – Temperatures increase with height. – – Oxygen and Nitrogen atoms absorbed shortwave, high energy radiation. Temperatures range from 500K to as high as 2000K depending on solar activity!!! ● Temperature a function of average molecular speed ● Very few molecules Meteorology 301 Zonal Averaged Temperature Meteorology 301 Tropospheric Winds at Middle and High Latitudes ● ● ● Winds tend to flow parallel to isobars or height contours with low pressure to the left (Northern hemisphere). Wind speed is inversely proportional to spacing of isobars or height contours. Geostrophic relationship. Meteorology 301 Average Zonal Wind Meteorology 301 Mean Geopotential Height Meteorology 301 Semi-Permanent Cells Meteorology 301 Tropospheric Winds at Low Latitudes ● Not as transient. ● Easterly flow. – ● Tropical cyclones (hurricanes) – ● Shift from north-east to south-east with weather systems. Strongest sustained winds observed anywhere on the earth's surface. ITCZ (Inter-Tropical Convergence Zone) Meteorology 301 Climatological-mean Precipitation (cm) (Be very careful with this scale.....not a very good choice) January July Meteorology 301 Scales in the atmosphere ● ● ● ● Planetary scale* – Horizontal dimensions comparable to scales of major oceans or continents. – Global circulation. Synoptic scale* – Few hundred kilometers. – Day to day weather Mesoscale* – Few 10km – few 100km – Jet streams, fronts, lee waves, rain bands squall lines, mid-latitude storms Microscale (small scale) – Everything smaller than mesoscale. Tornados, dust devils, individual storm cells, etc. Meteorology 301 Meteorology 301 Blizzard ● ● ● The following conditions are expected to prevail for a period of 3 hours or longer: Sustained wind or frequent gusts of 35mph or greater, and Considerable falling or blowing snow – Frequently reduces visibility to less than ¼ mile. Meteorology 301